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Differential therapeutic effects of atomoxetine and methylphenidate in childhood attention deficit/hyperactivity disorder as measured by near-infrared spectroscopy

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RESEARCH ARTICLE

Differential therapeutic effects

of atomoxetine and methylphenidate

in childhood attention deficit/hyperactivity

disorder as measured by near-infrared

spectroscopy

Yoko Nakanishi

1*

, Toyosaku Ota

1

, Junzo Iida

2

, Kazuhiko Yamamuro

1

, Naoko Kishimoto

1

, Kosuke Okazaki

1

and Toshifumi Kishimoto

1

Abstract

Background: The stimulant methylphenidate (MPH) and the nonstimulant atomoxetine (ATX) are the most com-monly-prescribed pharmacological treatments for attention deficit/hyperactivity disorder (ADHD). However, the drug-specific mechanism of action on brain function in ADHD patients is not well known. This study examined differences in prefrontal hemodynamic activity between MPH and ATX in children with ADHD as measured by near-infrared spectroscopy (NIRS) using the Stroop color-word task.

Methods: Thirty children with ADHD participated in the present study. We used 24-channel NIRS (ETG-4000) to measure the relative concentrations of oxyhemoglobin in the frontal lobes of participants in the drug-naïve condition and those who had received MPH (n = 16) or ATX (n = 14) for 12 weeks. Measurements were conducted every 0.1 s during the Stroop color-word task. We used the ADHD RS-IV-J (Home Version) to evaluate ADHD symptoms.

Results: Treatment with either MPH or ATX significantly reduced ADHD symptoms, as measured by the ADHD RS-IV-J, and improved performance on the Stroop color-word task in terms of number of correct words. We found signifi-cantly higher levels of oxyhemoglobin changes in the prefrontal cortex of participants in the ATX condition compared with the values seen at baseline (pre-ATX). In contrast, we found no oxyhemoglobin changes between pre- and post-treatment with MPH.

Conclusions: The present study suggests that MPH and ATX have differential effects on prefrontal hemodynamic activity in children with ADHD.

Keywords: Attention-deficit/hyperactivity disorder, Near-infrared spectroscopy, Functional neuroimaging, Atomoxetine, Methylphenidate

© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Background

Attention-deficit/hyperactivity disorder (ADHD) is one of the most commonly diagnosed neurodevelopmental disorders in children with lifelong deficits in a wide range

of executive functions [1]. ADHD symptoms are thought to arise from dysregulation of prefrontal and subcortical catecholamine neurotransmission [2, 3]. The stimulant methylphenidate (MPH) and the nonstimulant atom-oxetine (ATX) are the most frequently prescribed drugs for the treatment of ADHD. Both drugs are known to reduce clinical ADHD symptoms. The common mecha-nism of both drugs is that they modulate dopamine (DA) and norepinephrine (NE) neurotransmission [4]. Small

Open Access

*Correspondence: p-yoko@naramed-u.ac.jp

1 Department of Psychiatry, Nara Medical University School of Medicine, 840 Shijo-cho Kashihara, Nara 634-8522, Japan

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changes in DA or NE concentration affect networks of pyramidal cells in the prefrontal cortex (PFC), which regulates and sustains attention [5]. It is believed that the therapeutic effects of both medications occur primarily in the PFC [5], although the exact mechanisms of their actions are unclear.

Methylphenidate acts as an indirect DA agonist, inhib-iting DA reuptake by occupying the DA transporter [6]. MPH has also shown to block the norepinephrine (NE) transporter in NE transporter-rich regions, including the PFC [7]. In rodent studies, MPH has been shown to enhance the extracellular levels of both DA and NE [8]. In contrast, although ATX is a selective NE reup-take inhibitor, it also inhibits DA reupreup-take in the PFC. Therefore, while it does not increase DA in the striatum, it increases both DA and NE in the prefrontal cortex [8]. The partially overlapping pharmacologic profiles of these medications suggest both similarities and differ-ences in their therapeutic mechanisms of action. In the meta-analysis focused on the comparison between MPH and ATX, MPH showed a higher response rate com-pared to ATX [9]. In a randomized study directly com-paring MPH and ATX in adults with ADHD, the effects on executive functions were generally similar, although there was a suggestion that ATX might show a slight ben-efit to the immediate-release MPH in terms of improv-ing spatial plannimprov-ing [10]. However, another head-to-head study comparing the two drugs found that only osmot-ically-released MPH improved set-shifting and verbal fluency, although osmotically-released MPH and ATX both improved executive function generally in children and adolescents with ADHD [11]. Distinct underlying pharmacological mechanisms may cause these practi-cal differences. There are few neuroimaging studies that examined these differences [12, 13]. Cubillo et al. showed that ATX upregulated and normalized right dorsolateral prefrontal cortex under activation measured by func-tional magnetic resonance imaging (fMRI), while MPH upregulated left inferior frontal cortex activation [13].

Near-infrared spectroscopy (NIRS) enables the non-invasive detection of neural activity near the surface of the brain using near-infrared light [14, 15]. It measures alterations in oxygenated hemoglobin ([oxy-Hb]) and deoxygenated.

Hemoglobin ([deoxy-Hb]) concentrations in micro-blood vessels on the brain surface. Local increases in [oxy-Hb] and decreases in [deoxy-Hb] are indicators of cortical activity [15, 16]. In animal studies, [oxy-Hb] is the most sensitive indicator of regional cerebral blood flow because the direction of change in [deoxy-Hb] is determined by the degree of changes in venous blood oxygenation and volume [17]. Therefore, we decided to focus on changes in [oxy-Hb]. Furthermore, changes in

[oxy-Hb] have been associated with changes in regional cerebral blood volume, using a combination of NIRS and positron emission tomography (PET) measurements [18, 19]. NIRS is a neuroimaging modality that is espe-cially suitable for psychiatric patients for the following reasons [20]. First, because NIRS is relatively insensitive to motion artifact, it can be used in experimental sce-narios in which motion may occur, such as while assess-ing participants who are prone to vocalization. Second, participants can be examined in a natural sitting posi-tion, without any surrounding distractions such as fMRI. Third, the cost is much lower than that of other neuro-imaging modalities and the setup is very easy. Fourth, the high temporal resolution of NIRS is useful in char-acterizing the time course of prefrontal activity in peo-ple with psychiatric disorders [21, 22]. Fifth, functional studies of pediatric patients using single-photon emis-sion computed tomography (SPECT) and PET are rare due to restrictions regarding the use of radioactive mate-rials in young individuals. Accordingly, NIRS has been used to assess brain function in people with many types of psychiatric disorders, including schizophrenia, bipolar disorder, post traumatic disorder, obsessive–compulsive disorder, and ADHD [20–28].

In pediatric ADHD, reduced prefrontal hemody-namic response has been measured by NIRS [23, 29,

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be a differential hemodynamic response across MPH and ATX.

Methods

Participants

Thirty patients aged 6–14  years and diagnosed with ADHD according to the DSM-5 criteria [33] participated in the present study. Participants had no history of treat-ment for a developtreat-mental disorder, and had consulted an experienced pediatric psychiatrist at the Department of Psychiatry at Nara Medical University. These partici-pants underwent a standard clinical assessment compris-ing a psychiatric evaluation, a semi-structured diagnostic interview (the kiddie schedule for affective disorders and schizophrenia for school-age children-present and life-time version [34]), and a medical history assessment. Two experienced pediatric psychiatrists confirmed the diagnosis of ADHD according to the DSM-5 criteria [33]. Intellectual level was assessed using the Wechsler intel-ligence scale for children-fourth edition (WISC-IV), and individuals with full-scale IQ (FIQ) scores below 70 were excluded. We also excluded those who presented with a comorbid Axis I diagnosis, a neurological disorder, a head injury, a serious medical condition, or a history of substance abuse/dependence because these influenced the prefrontal hemodynamic response [20–22, 24, 26,

35, 36]. In total, 30 participants with ADHD who had no previous medication history were enrolled in the present study. All participants were right-handed and of Japanese descent.

We used NIRS to measure the relative concentra-tions of oxy-Hb in participants in the drug-naïve con-dition (pre-treatment) and after 12  weeks of treatment with either osmotically released MPH (n = 16) or ATX (n = 14) (post-treatment). The participants were assigned either MPH or ATX by the decision of an experienced pediatric psychiatrist. All measurements were conducted at the same time of day (10.00–11.00 h). All the partici-pants were MPH and ATX naïve and started to take MPH 18 mg/day or ATX 0.5 mg/kg/day in the morning, respec-tively. They were titrated up as needed to the lowest effective dose by the decision of an experienced pediatric psychiatrist every 2 weeks. The mean dose of MPH was 0.87 mg/kg (SD = 0.23), and the mean dose of ATX was 1.30  mg/kg (SD =  0.44). The characteristics of the par-ticipants are shown in Table 1. This study was approved by the Institutional Review Board at Nara Medical Uni-versity. Written informed consent was obtained from all participants and/or their parents prior to the study.

Assessment of ADHD symptoms

We used the ADHD Rating Scale-IV-Japanese version (ADHD RS-IV-J) (Home Version) [37] to evaluate ADHD

symptoms in the participants. A higher ADHD RS-IV-J score is associated with more severe ADHD symptoms. All participants underwent ADHD RS-IV-J assessment pre- and post-treatment which were rated by parents (Table 3).

The Stroop color‑word task

The traditional Stroop task was combined with the word-reading task, incongruent color-naming task, and the color-naming task. However, we reconstructed the Stroop task according to previously described methods [38]. The Stroop color-word task consisted of two pages stapled together: each page had 100 items in five columns of 20 items each and the page size was 210 × 297 mm. On the first page, the words RED, GREEN, and BLUE were printed in black ink. On the second page, the words RED, GREEN, and BLUE were printed in red, green, or blue ink, with the limitation that the word meaning and ink color could not match. The items on both pages were randomly distributed, with the exception that no item could appear directly after the same item within a col-umn. Before the task, the examiners instructed the par-ticipants as follows: ‘This is to test how quickly you can read the words on the first page, and say the colors of the words on the second page. After we say “begin”, please read the words in the columns, starting at the top left, and say the words/colors as quickly as you can. After you finish reading the words in the first column, go on to the next column, and so on. After you have read the words on the first page for 45 s, we will turn the page. Please repeat

Table 1 Participant characteristics

MPH methylphenidate, ATX atomoxetine, NA not applicable, FIQ full-scale IQ,

WISC-IV Wechsler Intelligence Scale for children-fourth edition, ARF ADHD RS IV-J full scores, ARI ADHD RS IV-J inattention subscale scores, ARH ADHD RS IV-J hyperactivity subscale scores, SCWC-1 Stroop color-word task number of correct answers first time, SCWC-2 Stroop color-word task number of correct answers second time, SCWC-3 Stroop color-word task number of correct answers third time

a The Chi square test was used; otherwise t-tests were used

MPH (n = 16) ATX (n = 14) p value Mean SD Mean SD

Sex (male/female)a 14/2 11/3 0.642

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this procedure for the second page.’ The entire Stroop color-word task sequence consisted of three cycles of 45 s spent reading the first page and 45  s spent reading the second page (the color-word task). The task ended with 45  s spent reading the first page, which we designated as the baseline task. We recorded the number of correct answers in each cycle, and refer to them as follows:

Stroop color-word task number of correct answers first time (SCWC-1), second time (SCWC-2), and third time (SCWC-3). Examiners who were blind to the diagnoses of the participants administered the Stroop color-word task. The Stroop task used in this study was different from the traditional Stroop task. We made the Stroop color-word task simple because the participants were school-aged children. Furthermore, we excluded the color-naming task (part of the traditional Stroop task) because we wanted to have only two tasks (baseline task and activation task) for our NIRS study.

NIRS measurements

We measured [oxy-Hb] using a 24-channel NIRS machine (Hitachi ETG-4000, Hitachi Medical Corporation, Tokyo, Japan). We measured the absorption of two wavelengths of near-infrared light (760 and 840  nm). [Oxy-Hb] was calculated as previously described [39]. The inter-probe intervals of the machine were 3.0  cm, and previous reports have established that the machine measures at a point 2–3 cm beneath the scalp, that is, the surface of the cerebral cortex [36, 40]. The participants were asked to adopt a natural sitting position for the NIRS measure-ment. The distance between the participants’ eyes and the paper on which items were listed was between 30 and 40  cm. The NIRS probes were placed on the scalp over the prefrontal brain regions, and arranged to measure the relative changes in Hb concentration at 24 measurement points that made up an 8 ×  8-cm2. The lowest probes

were positioned along the Fp1–Fp2 line according to the international 10/20 system commonly used in electroen-cephalography. The correspondence between the probe positions and the measurement points in the cerebral cortex were confirmed by superimposing the probe posi-tions onto a three-dimensionally reconstructed cerebral cortex of a representative participant in the control group, obtained via MRI (Fig. 1). The absorption of near-infrared light was measured with a time resolution of 0.1  s. The data were analyzed using the ‘integral mode’: the pre-task baseline was determined as the mean across the 10 s just before the task period, the post-task baseline was deter-mined as the mean across the 25 s immediately after the task period, and linear fitting was performed on the data between the two baselines. Moving average methods were used to exclude short-term motion artifacts in the ana-lyzed data (moving average window, 5 s). We attempted

to exclude motion artifacts by closely monitoring artifact-evoking body movements, such as neck movements, bit-ing, and blinking (identified as being the most influential in a preliminary artifact-evoking study), and by instruct-ing the participants to avoid these movements durinstruct-ing the NIRS measurements. Examiners were blind to the treat-ment condition of the participants.

Statistical analysis

We used the Chi square (χ2) test to examine group

differ-ences for categorical variables (e.g. gender). Clinical vari-ables with a normal distribution were compared using Student’s t tests. Correlations between SCWC and char-acteristics of the subjects were tested with Spearman’s correlation test. For statistical comparison of the partici-pant characteristics between the pre- and post-treatment conditions, we used a two-tailed paired t test. Specifi-cally, we compared oxy-Hb changes between the pre- and post-treatment conditions. To conduct a more detailed comparison of oxy-Hb changes along the time course of the task, we used MATLAB 6.5.2 (Mathworks, Natick, MA, USA) and Topo Signal Processing type-G version 2.05 (Hitachi Medical Corporation, Tokyo, Japan).

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The threshold for statistical significance was set at p < 0.05. Bonferroni-adjusted p values are reported (i.e. corrected for multiple comparisons). We used PASW Statistics18.0J for Windows (SPSS, Tokyo, Japan) for statistical analyses.

Results

Demographic data

The demographic characteristics of the study participants are presented in Table 1. The participant groups did not differ in terms of mean age, sex, FIQ, ADHD-RS-IV-J scores including the ARF, ARI and ARH subscale scores, and SCWC-3 scores (p  >  0.125 for all 7 variables). We found significant differences in the SCWC-1, SCWC-2 scores between the MPH and ATX groups (t = −2.52, p = 0.018; t = −2.53, p = 0.017).

Correlation between Stroop task performance and participant characteristics

Because the MPH and ATX groups varied consider-ably in terms of SCWC-1 SCWC-2 scores, we calculated

Spearman’s correlations for the SCWC scores, age, FIQ, and ADHD-RS-IV-J, as shown in Table 2. In the ATX group, the SCWC-1, SCWC-2 and SCWC-3 scores were positively correlated with age (ρ = 0.866, p < 0.000, ρ = 0.798, p < 0.001 and ρ = 0.718, p < 0.004), while none of SCWC scores significantly correlated with FIQ and ADHD-RS-IV-J scores. In the MPH group, the SCWC2 score were positive correlated with age (ρ =  0.522, p  <  0.038), and SCWC1 score were positive correlated with FIQ (ρ =  0.557, p  <  0.025), whereas none of the SCWC scores were significantly associated with ADHD-RS-IV-J scores.

Clinical and behavioral improvement

Both treatments were associated with statistically sig-nificant improvements in terms of both ADHD-RS-IV-J scores and SCWC scores, as shown in Table 3. In both groups, the ADHD-RS-IV-J scores including the ARF, ARI and ARH subscale scores in the post-treatment condition were significantly lower than scores in the

Table 2 Correlation between Stroop task performance and participant characteristics

MPH methylphenidate, ATX atomoxetine, FIQ full-scale IQ, WISC-IV Wechsler Intelligence Scale for children-fourth edition, ARF ADHD RS IV-J full scores, ARI ADHD RS IV-J inattention subscale scores, ARH ADHD RS IV-J hyperactivity subscale scores, SCWC-1 Stroop color-word task number of correct answers first time, SCWC-2 Stroop color-word task number of correct answers second time, SCWC-3 Stroop color-word task number of correct answers third time

* p < 0.05 ** p < 0.01

MPH (n = 16) ATX (n = 14)

SCWC‑1 SCWC‑2 SCWC‑3 SCWC‑1 SCWC‑2 SCWC‑3

Age 0.483 0.522* 0.417 0.866** 0.798** 0.718** FIQ 0.557* 0.304 0.307 −0.159 −0.023 0.982 ARF −0.368 −0.292 −0.351 −0.248 −0.237 −0.306 ARI −0.001 −0.076 −0.162 −0.085 −0.116 −0.227 ARH −0.448 −0.364 −0.335 −0.422 −0.376 −0.402

Table 3 Clinical outcome and task performance

MPH methylphenidate, ATX atomoxetine, ARF ADHD RS IV-J full scores, ARI ADHD RS IV-J inattention subscale scores, ARH ADHD RS IV-J hyperactivity subscale scores,

SCWC-1 Stroop color-word task number of correct answers first time, SCWC-2 Stroop color-word task number of correct answers second time, SCWC-3 Stroop color-word task number of correct answers third time

a Two-tailed paired t test b Two-way factorial ANOVA

MPH (n = 16) ATX (n = 14) p value

Pre‑ Post‑ Pre‑ Post‑ MPH ATX

Mean (SD) Mean (SD) Mean (SD) Mean (SD) Pre vs posta Pre vs posta Time × Drug Interactionb 

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pre-treatment condition (p  <  0.01 for all 6 variables). Additionally, the SCWC-1, SCWC-2 and SCWC-3 scores in the post-treatment condition were significantly higher than those in the pre-treatment condition (p < 0.033 for all 6 variables). There were no significant main effects of treatment condition × medication interactions for any of the performance measures (p > 0.098 for all 6 variables).

Comparison of NIRS measurements between pre‑ and post‑ treatment

We calculated the grand average waveforms of [oxy-Hb] concentration changes during the Stroop color-word task in the pre- and post-treatment condition (Figs. 2, 3). In the MPH group, the grand waveforms of [oxy-Hb] con-centration showed little change in both pre- and post-conditions (Fig. 2). We did not find any differences in mean [oxy-Hb] measurements between pre- and post-MPH in any of the 24 channels that were recorded in Table 4. By contrast, in the ATX group, the grand wave-forms of [oxy-Hb] concentration change appeared to increase substantially during task performance in the post- rather than in the pre-condition (Fig. 3). On chan-nel 21, the mean oxy-Hb measurement was significantly larger in the post-condition relative to the pre-condition, as displayed in Table 5.

Comparison of NIRS measurements between two groups Channel 21 showed significant treatment-by-condition interactions (F = 13.102, p = 0.002). However, there were no main effects for either treatment or condition on chan-nel 21 (F = 2.260, p = 0.147; F = 3.99, p = 0.058). We

did not find any differences in mean [oxy-Hb] measure-ments between the pre-ATX and the pre-MPH (t = 0.756, p =  0.458) on this channel. However, the mean oxy-Hb measurement for channel 21 was significantly larger for post-ATX relative to post-MPH (t = −0.2802, p = 0.009).

Correlation between degree of clinical improvement and hemodynamic change in Channel 21

We conducted Spearman’s rank correlation analy-ses between hemodynamic change in channel 21 with scores of SCWC and ADHD-RS-IV-J scores, shown in Table 6. There were no correlations between hemody-namic change and these scores for either ATX or MPH (all p > 0.2).

Discussion

To our knowledge, this is the first NIRS study to com-pare the effectiveness of MPH with ATX directly in chil-dren with ADHD by measuring hemodynamic responses during the Stroop color-word task. ATX significantly increased activation in the prefrontal cortex, especially in left lateral frontal pole cortex (FPC), after 12 weeks of administration. MPH did not increase activation in the prefrontal cortex, but it did make comparable improve-ments in terms of ADHD symptoms and Stroop color-word task performance to those seen in ATX.

Some studies have referred indirectly to differences in the neurobiological actions between MPH and ATX. Event-related potential studies of oddball tasks in pedi-atric ADHD have shown that MPH can normalize low Fig. 2 Grand average waveforms showing changes in

oxyhemoglobin(oxy-Hb) during the Stroop color-word task pre- and post-MPH. Cyan lines indicate pre-MPH and blue lines indicate post-MPH. Yellow lines indicate the beginning and end of each trial. Ch

channel

Fig. 3 Grand average waveforms showing changes in

oxyhemoglobin(oxy-Hb) during the Stroop color-word task pre- and post-ATX. Pink lines indicate pre-ATX and red lines indicate post-ATX. Yellow lines indicate the beginning and end of each trial. The statistically significant region is shown within navy frames (Ch21). Ch

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P300 or mismatch negativity amplitudes [41], while ATX can normalize long P300 latencies and low MMN ampli-tudes, at least partially [42]. In a fMRI study of adult ADHD using a multi-source interference task, ATX did not activate dorsal anterior midcingulate cortex [43], as MPH has been shown to do [44]. However, few stud-ies have directly investigated how the pharmacologi-cal mechanisms of action differ between the two drugs, and little is known about the mechanisms by which they exert their therapeutic effects. In one fMRI study that used a go/no-go task with 36 participants with pediatric ADHD, comparable improvements in response inhibi-tion and ADHD symptoms were seen after 6 to 8 weeks of daily treatment with MPH vs ATX. Symptomatic improvement was associated with gains in task-related activation for ATX and reductions in activation for MPH in the right inferior frontal gyrus, left anterior cingulate/ supplementary motor area, and bilateral posterior cin-gulate cortex [45]. In another fMRI study using a count-ing Stroop paradigm, 12 weeks of ATX pharmacotherapy

decreased activity in the dorsal anterior cingulate cortex and dorsolateral prefrontal cortex in 42 participants with pediatric ADHD, which correlated with improvement in focused attention. In contrast, MPH increased activity in the inferior frontal gyrus, which correlated with decreas-ing severity of impulsivity [46]. Comparing effects of acute doses of both drugs and a placebo with boys with ADHD during a stop task, MPH had a drug-specific effect of normalizing the right ventrolateral prefrontal and cer-ebellar under-activation observed under both placebo and ATX [47]. Taken together, these reports indicate that the mechanisms by which MPH and ATX exert their thera-peutic effects are different: this is consistent with the find-ings from the present study. Nevertheless, the concept of drug-specific laterality effects on prefrontal regions is still controversial. Our data showed that ATX upregulated the frontal cortex during Stroop interference, at least partially. The present findings suggest that frontal mechanisms serve an important role in the therapeutic actions of ATX. However, despite the fact that MPH did not increase acti-vation in the PFC, there were still comparable improve-ments in terms of ADHD symptoms for those taking this medication. One parsimonious explanation is that MPH increases activation in other brain regions, which might contribute to the improvement in ADHD symptoms.

Volkow et al. [48, 49] found that in healthy adults, MPH enhanced the salience of a reward task, increased levels of extra-cellular dopamine, and induced reductions in glu-cose metabolism within the default mode network (DMN). The DMN is a distributed brain system, comprising medial pre-frontal cortex and medial and lateral parietal regions. It is anti correlated with the attentional networks acti-vated by goal-directed behavior, and is thought to reflect intrinsic activity [50]. Recently, influential new brain net-work models [51, 52] have proposed that proper sustained attention functioning requires both engagement of task-positive networks (TPNs), including a frontoparietal con-trol network and dorsal and ventral attention networks, and suppression of the DMN [50, 53, 54]. A failure of the anti-phase synchronization between DMN and TPN may be involved in the manifestation of ADHD. There is evi-dence suggesting that the striatal DA system plays a role in the modulation of the DMN [55, 56]. MPH produces robust increases in extracellular dopamine levels [57], which potentiate corticostriatal inputs [58] and have been found to enhance striatal activation in child ADHD [59,

60]. Furthermore, some studies have shown that MPH may normalize DMN deactivation patterns [61, 62]. There-fore, we speculate that MPH might tend to activate DMN regions rather than TPN during task-related activation. In contrast, an increase of prefrontal activation has been reported after MPH treatment in several studies using dif-ferent neuroimaging modalities, including NIRS [44, 59,

Table 4 Difference in  mean oxyhemoglobin between  the task and post-task periods pre- and post-MPH

Group differences tested with t test

NS not significant

Pre‑MPH (mMmm) Post‑MPH (mMmm) Student’s t test

Mean SD Mean SD

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63, 64]. The variability in findings across studies is likely related to different cognitive tasks, dosage, patients’ ages, and treatment duration.

Increases in left lateral FPC activity were observed after ATX treatment in our study. However, we found no significant correlations between the hemodynamic changes in this area and degree of the clinical improve-ments. The FPC is the most anterior part of the cerebral cortex, and has reciprocal connections with most pre-frontal areas [65, 66]. Tsujimoto et al. suggested that the FPC has a role in monitoring and evaluating decisions, especially those with a self-generational component [67]. Arai et  al. found that children with ADHD show abnormalities in functional maturation of the frontal pole [68]. Based on these findings, a direction for future research will be to assess participants using another bat-tery associated with self-generated behavior, separate to NIRS recordings.

The results of the present study suggest that multi-channel NIRS systems may have potential in the phar-macotherapeutic evaluation in children with ADHD for clinical practice. It is very significant for patients that an effect of the pharmacotherapy is visualized. In the future,

Table 5 Difference in mean oxyhemoglobin measurements between the task and post-task periods pre- and post-ATX

Group differences tested with t test

NS not significant

* Significant with Bonferroni correction for multiple comparisons

Pre‑ATX (mMmm) Post‑ATX (mMmm) Student’s t test Bonferroni correction

Mean SD Mean SD

Ch1 0.013 0.0717 −0.0389 0.0706 NS NS Ch2 −0.0092 0.1313 −0.0309 0.0681 NS NS

Ch3 0.0294 0.0837 0.0194 0.0848 NS NS

Ch4 0.0259 0.0595 0.018 0.0571 NS NS

Ch5 −0.0008 0.0656 −0.0191 0.0542 NS NS

Ch6 0.0213 0.0789 0.0000 0.0477 NS NS

Ch7 −0.0209 0.0634 0.0133 0.0821 NS NS

Ch8 0.0107 0.0767 0.0508 0.0761 NS NS

Ch9 −0.0049 0.0734 −0.0095 0.0968 NS NS Ch10 −0.0182 0.0419 −0.0312 0.0667 NS NS Ch11 0.0042 0.0941 0.0483 0.0747 NS NS Ch12 −0.0337 0.0558 −0.0159 0.0621 NS NS Ch13 −0.0363 0.0865 −0.0059 0.0565 NS NS Ch14 −0.0039 0.0695 0.0042 0.0731 NS NS Ch15 −0.0278 0.0949 0.0061 0.084 NS NS Ch16 −0.0119 0.1077 0.0109 0.0606 NS NS Ch17 −0.0013 0.057 0.0314 0.0438 NS NS

Ch18 0.046 0.0746 0.0456 0.1136 NS NS

Ch19 −0.0391 0.0727 0.0178 0.0652 p = 0.049 NS Ch20 0.0585 0.0871 0.0883 0.1638 NS NS Ch21 −0.0341 0.0753 0.0956 0.0954 p = 0.000 *

Ch22 0.006 0.0844 0.0156 0.0813 NS NS

Ch23 −0.0101 0.0803 0.0202 0.0899 NS NS Ch24 0.006 0.101 −0.0123 0.0773 NS NS

Table 6 Correlation between  degree of  clinical improve-ment and hemodynamic change in channel 21

Tested using Spearman’s correlation test

MPH methylphenidate, ATX atomoxetine, ARF ADHD RS IV-J full scores, ARI ADHD RS IV-J inattention subscale scores, ARH ADHD RS IV-J hyperactivity subscale scores, SCWC-1 Stroop color-word task number of correct answers first time,

SCWC-2 Stroop color-word task number of correct answers second time, SCWC-3

Stroop color-word task number of correct answers third time All p > 0.216

MPH ATX

ARF −0.006 −0.196

ARI −0.072 −0.406

ARH −0.017 −0.087

SCWC-1 −0.331 0.147

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it is need to predict the effect of the pharmacotherapy using the NIRS for clinical practice.

The present study has several potential limitations. First, methodological limitations include the relatively small number of participants, non-randomised study, and lack of a double-blind, placebo-controlled design. At baseline, the ATX group had higher mean SCWC1 and SCWC2 scores than the MPH group. Although scores were not correlated with degree of clinical severity with ADHD-RS, the two groups were not quite entirely equivalent in their charac-teristics. Future work seeking to compare MPH, ATX and/ or placebo should consider a double-blind randomized or crossover design with larger samples. Second, we had no healthy control as a comparison cohort. Our study showed that ATX significantly increased activation in channel 21. In one previous NIRS study using the Stroop, Negoro et al. reported a lower increase of oxy-changes in channels 8, 18, 19, 21, and 22 in individuals with child ADHD com-pared with controls [23]. Considering the above findings, we predicted that improvement of ADHD symptoms with ATX treatment would be associated with increased acti-vation in those regions; our findings were consistent with these predictions. Third, NIRS does not detect activity in deeper cortical structures, such as the medial pre-frontal cortex, which is part of the DMN. Fourth, the spatial reso-lution for the detection of hemodynamic responses from the scalp surface using NIRS is lower than that of fMRI, SPECT, or PET. However, the spatial resolution may be within an acceptable range because previous NIRS stud-ies have also found clear distinctions in hemodynamic responses between diagnostic groups [23–26, 28, 69].

Conclusions

In conclusion, this is the first NIRS study using the Stroop interference task to examine how the pharmacological mechanisms of action differ between MPH and ATX. Find-ings suggest that effective treatment with MPH and ATX is produced by distinct mechanisms in frontal regions.

Abbreviations

MPH: methylphenidate; ATX: atomoxetine; ADHD: attention deficit/hyperactiv-ity disorder; fMRI: functional magnetic resonance imaging; NIRS: near-infrared spectroscopy; DA: dopamine; NE: norepinephrine; PFC: prefrontal cortex; SPECT: single-photon emission computed tomography; PET: positron emis-sion tomography; FIQ: full-scale IQ; WISC-IV: Wechsler intelligence scale for children-fourth edition; ARF: ADHD RS IV-J full scores; ARI: ADHD RS IV-J inat-tention subscale’s scores; ARH: ADHD RS IV-J hyperactivity subscale’s scores; SCWC-1: Stroop color-word task number of correct answers first time; SCWC-2: Stroop color-word task number of correct answers second time; SCWC-3: Stroop color-word task number of correct answers third time; FPC: frontal pole cortex; DMN: default mode network; TPNs: task-positive networks.

Authors’ contributions

YN, TO and JI developed the study protocol. YN, TO, KY, NK and KO performed the experiments. YN and TO analyzed the data. JI and TK administered and supervised programs and overall conduct of the study. All authors read and approved the final manuscript.

Author details

1 Department of Psychiatry, Nara Medical University School of Medicine, 840 Shijo-cho Kashihara, Nara 634-8522, Japan. 2 Faculty of Nursing, Nara Medical University School of Medicine, 840 Shijo-cho Kashihara, Nara 634-8522, Japan.

Acknowledgements

We wish to thank the participants for their invaluable participation in the study. The authors would also like to thank Hitachi Medical Corporation for the ETG-4000 equipment, and the skilled technical and methodical support.

Competing interests

YN, TO, KY, NK and KO have no competing interests, respectively. JI and TK have received honoraria for lecturing from Eli Lilly and Janssen Pharmaceutica.

Consent for publication

Written informed consent was obtained from all participants and/or their parents prior to the study.

Ethics approval and consent to participate

This study was approved by the Institutional Review Board at Nara Medical University.

Funding

This study was supported by JSPS KAKENHI Grant Number 22591285 without a further role in the study design, collection, analysis, and interpretation of the data, drafting the report, or the decision to submit the paper for publication.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in pub-lished maps and institutional affiliations.

Received: 4 December 2016 Accepted: 6 May 2017

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Figure

Table 1 Participant characteristics
Fig. 1 Location of the 24 channels on the near-infrared spectroscopy instrument
Table 2 Correlation between Stroop task performance and participant characteristics
Table 4. By contrast, in the ATX group, the grand wave-forms of [oxy-Hb] concentration change appeared to increase substantially during task performance in the
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